The invention relates to a process for preparing highly sulfurized molybdenum dithiocarbamates. Conventional sulfurized molybdenum dithiocarbamates are known for use in lubricating formulations, and are currently used as additives in lubricating oils for imparting antifriction and antiwear properties, as well as extreme pressure and antioxidant properties. These compounds have the general structure [R1R2N—CS—S]2 Mo2OxS4-x, where x=2.35-3 and, therefore, are not highly sulfurized.
It is also known to increase the sulfur to oxygen in this structure [R1R2N—CS—S]2 Mo2OxS4-x, where x=0.75 to 2.1 and R1 and R2, which are the same or different, are hydrocarbyl groups containing 1 to 24 carbon atoms. Such higher sulfur compositions are prepared from a sulfide compound such as alkaline metal hydrogen sulfides, ammonium hydrogen sulfide, alkaline metal sulfides, ammonium sulfide and mixtures thereof as taught in U.S. Pat. No. 4,098,705.
There is a desire for the dithiocarbamates to have higher sulfur content (higher sulfurized). With sulfur being an antioxidant, these compounds have better antioxidant stability afforded by the increased sulfur content. Very high sulfur levels have been found to produce copper corrosion, but levels of up to about three sulfur atoms on the molybdenum core of structure [R1R2N—CS—S]2 Mo2OxS4-x, where x≧1.0, are acceptable. High sulfur content (wherein sulfur replaces oxygen) affords an internal antioxidant at the molecular level, which provides an increased stability to the molecule, which is believed to afford improved antifriction retention properties in their application. The known preparation methods for the higher sulfurized molybdenum dithiocarbamates involve the use of hydrogen sulfide or other sulfide during preparation to exchange oxygen-bound molybdenum to sulfur-bound molybdenum. The drawback of this process is the use of hydrogen sulfide or sources thereof, e.g. alkaline metal sulfides, ammonium sulfide and alkaline metal hydrogen sulfides, due to the danger and difficulty of handling these highly toxic materials.